A PCR sequencing method refers to a technique wherein DNA fragments of a gene of interest are obtained by a PCR method, and the obtained DNA fragments of the gene of interest are subjected to DNA sequencing to obtain the DNA sequence information of the gene of interest. PCR sequencing methods are widely applied to the fields such as detection of gene mutation and genotyping for a long time.
DNA sequencing technique is mainly classified into the first generation DNA sequencing technique represented by Sanger sequencing method and the second generation DNA sequencing technique represented by Illumina GA, Roche 454, ABI Solid, and the like. Sanger DNA sequencing technique is characterized by simple experimental operations, visual and accurate results, and short experimental period, and thus is wildly applied in fields such as clinical gene mutation detection and genotyping, wherein a fast turnaround time is highly required as to detection results. However, due to the characteristics such as low throughput and high cost, its application in fields where genotyping is performed in a large scale, is limited.
As compared with the first generation DNA sequencing technique, the second generation DNA sequencing technique has the characteristics such as high sequencing throughput, low cost, high level of automation, and single-molecule sequencing. Taken Illumina GA single-molecule sequencing as an example, a single sequencing run generates data of 50 G (about 50 billion) bases, 5 billion bases data per day in average, and the average sequencing cost for a base is less than 1/1000 of the sequencing cost in Sanger method. Moreover, the analysis of results can be directly carried out by a computer. Thus, the second generation DNA sequencing technique is a technique quite suitable for large-scale sequencing projects. However, the contiguous sequencing length is generally short in the second generation DNA sequencing technique. Currently, the maximum bidirectional sequencing length is 200 bp for Illumina GA; although the maximum sequencing length can be up to about 500 bp for Roche 454 GS-FLX, the sequencing cost is high and the throughput is low. When a PCR amplicon is of a length greater than the maximum sequencing length in a sequencer, the thorough sequencing of the whole amplicon cannot be accomplished by sequencing directly, and the whole DNA sequence information of the amplicon cannot be obtained. Due to short maximum sequencing length, the application of the second generation sequencing technique in PCR sequencing method is limited. In addition to gradual improvement of sequencing technique to obtain a longer maximum sequencing length, it is urgent need to develop a new technique to overcome the deficiency of the current maximum sequencing length of the second generation DNA sequencer in the PCR sequencing application field.
Human leukocyte antigen (HLA) is one of the gene systems found so far to be of the highest polymorphism. It is a primary gene system for modulating specific immune response in human bodies and determining individual difference in susceptibility to diseases, and is closely associated with allogeneic organ transplant rejection. It is found in studies that the higher the matching degree of genes, such as HLA-A, B, C, DRB1 and DQB1, as well as the resolution are in a donor and a receptor, the longer a transplant survives. It is already a regular testing item to subject a potential donor and a receptor to high-resolution HLA genotyping before hematopoietic stem cell transplantation.
The current international standard HLA high-resolution genotyping technique is a Sanger sequencing technique-based PCR sequencing method, which comprises PCR amplifying the corresponding HLA gene regions, sequencing the amplified product, subjecting the sequencing result to genotyping with a professional genotyping software, and finally obtaining the HLA genotype information of the sample. It is characterized by visual results, high resolution and capability of detecting new allele. However, due to the characteristics of Sanger sequencing, such as high cost and low throughput, its application in institutes like hematopoietic stem cell volunteer registration database (Bone Marrow Bank), in which large-scale HLA genotyping detection is required, is limited.
It was reported that a Roche 454 GS-FLX-based PCR sequencing method was used in HLA genotyping. However, since its cost for sequencing was relatively high, it was not significantly superior over the Sanger sequencing-based HLA genotyping technique in terms of sequencing throughput and sequencing cost. As compared with Roche 454 GS-FLX, Illumina GA has a shorter maximum sequencing length, but has obvious advantages in terms of sequencing throughput and sequencing cost. If the defect of the short maximum sequencing length of Illumina GA can be overcome, its application in HLA genotyping will make up for the shortage of the current HLA genotyping method.